The present invention relates to a method and apparatus for the production of droplets of liquid at high temperatures, such as a molten metal or alloy, and more particularly the present invention relates to a method and apparatus for controlled generating of droplets on demand in manufacturing processes.
Free standing metal objects can be manufactured by the deposition of individual droplets of molten metal, using a computer to manipulate the droplet generator and the substrate, as described in U.S. Pat. No. 5,340,090 to Orme et al. and U.S. Pat. No. 5,746,844 to Sterett. Droplet deposition has also been used in U.S. Pat. No. 5,229,016 to Hayes et al. to dispense small amounts of solder at precisely determined locations on a circuit board prior to attaching an integrated circuit chip to it. Such manufacturing techniques require the ability to generate, on demand, small droplets of a molten metal. Consequently several designs for such droplet generators have been developed. Typically such generators consist of a heated chamber filled with molten metal. A droplet is formed by applying a pressure pulse to the pool of metal, ejecting a small quantity of liquid through a nozzle. Several different techniques have been used to apply this pressure pulse, including piezoelectric crystals, mechanical plungers, acoustic waves, and magneto-hydrodynamic (MHD) forces discussed herebelow.
Piezoelectric droplet generators are widely used for ink-jet printing. They have a chamber containing liquid, one wall of which is made from piezoelectric material. Applying a voltage pulse to the piezo-electric crystal makes it flex, sending a pressure pulse through the liquid in contact with it and forcing out a droplet. In U.S. Pat. No. 4,828,886 to Hieber, liquid solder is supplied through a glass tube around which an annular piezoelectric transducer is mounted. Application of a voltage to the transducer makes it contract and compress the glass tube, emitting solder from the tube. Use of such transducers is restricted to low melting point metals, because they lose their responsive properties above the Curie temperature (about 350xc2x0 C. for most piezoelectric materials).
Mechanical systems of levers and plungers have been used to form droplets of high temperature metals. Chun et al. (U.S. Pat. No. 5,266,098) and Yuan et al. (U.S. Pat. No. 5,609,919) used a reciprocating plunger to periodically apply impulses to a liquid metal and force it through an array of holes in the bottom of the container. In U.S. Pat. No. 5,598,200 to Gore single droplets are ejected on demand by positioning a plunger over an orifice in a chamber containing a liquid, and rapidly moving the plunger towards the orifice. Mechanical actuators allow droplet generators to be used at high temperatures, but increase their complexity and restrict the frequency with which droplets can be produced.
Acoustic radiation pressure can be used to eject metal droplets from the free surface of a pool of molten metal by directing towards the surface bursts of energy from an acoustic source located at the bottom of the pool (U.S. Pat. No. 5,722,479 to Oeftering). Magneto-hydrodynamic (MHD) forces can also be used (U.S. Pat. No. 4,919,335 to Hobson et al.) to form a fine spray by passing an electric current through the molten metal and simultaneously applying a magnetic field perpendicular to the direction of the electric current. The resultant MHD force is used to force molten metal through a nozzle, forming droplets. Acoustic and MHD droplet generators are useful in producing sprays, but it is difficult to precisely control the size of droplets produced by these devices.
A stream of droplets can be produced by vibrating a liquid jet issuing from an orifice, inducing capillary instabilities that break the stream into uniform sized droplets. The excitation force can be applied to the jet using either an acoustic source (as in U.S. Pat. No. 5,445,666 to Peschka et al.) or a mechanical actuator (as in U.S. Pat. No. 5,810,988 to Smith Jr. et al.). This technique is useful in forming metal micro-spheres, but cannot be used to generate droplets on demand.
It would be very beneficial to provide a method and device for reproducibly producing individual droplets of a chosen size.
The present invention provides a method and device for producing individual or multiple droplets of a chosen size on demand.
In one aspect of the invention there is provided a method of producing droplets, comprising pressure pulsing a chamber with a gas, the chamber holding a material to be ejected as droplets, the chamber being pressurized for a sufficient time to build up a pressure sufficient to forcefully eject at least one droplet of said material through an outlet and thereafter relieving the pressure sufficiently rapidly to avoid ejection of further droplets from the chamber.
In another aspect of the invention there is provided an apparatus for generating and ejecting droplets therefrom, comprising:
a) a housing enclosing a chamber for holding a material to be ejected therefrom, a gas inlet and an outlet passageway communicating with said chamber;
b) pressurizing means connected to said gas inlet for pressure pulsing the chamber with a gas for forcefully ejecting at least one droplet through said outlet passageway; and
c) pressure relief means for relieving pressure in said chamber sufficiently rapidly to avoid ejection of further droplets and to provide control of a number of droplets ejected from said chamber through said outlet passageway.
In this aspect of the invention the pressure relief means may include a vent in communication with the chamber for relieving pressure in the chamber, the vent having an effective size so that during application of a gas pulse the chamber is pressurized to a pressure sufficient to eject a droplet of material therefrom and thereafter the chamber is vented through the vent at a rate sufficient to prevent further discharge of droplets.